Cystamine and cysteine inhibited the carbon tetrachloride (CCl4) prooxidant effect on rat liver microsomal preparations, at concentrations ranging from 0.001 mM to 1 mM. Cysteine exhibited a biphasic effect being an inhibitor of the prooxidant action at concentrations below 0.1 mM and acting as an enhancer at 1 mM. Cystamine but not cysteine pretreatment of the rats prevented the CCl4 induced decrease in the arachidonic acid content of liver microsomal phospholipids. However, both cystamine and cysteine led to decreases in arachidonic acid similar to that produced by CCl4 but they do not have deleterious effects on the liver. These results cast doubt on the role of lipid peroxidation in the liver cell injury by CCl4.